A person who congratulates; someone who expresses joy or praise for another's achievement or good fortune.
From Latin 'congratulator' (one who wishes joy together), using the agent suffix '-or' to indicate a person who performs the action of congratulating.
While 'congratulant' and 'congratulator' both mean 'someone who congratulates,' 'congratulator' is the more officially recognized version in dictionaries, showing how English sometimes keeps multiple forms.
Complete word intelligence in one call. Free tier — 50 lookups/day.